NiMoO4 nanoflowers on nickel foam as electrocatalysts for water oxidation

ABSTRACT

A rapid method of synthesizing nanoflowers made of nanoflakes of nickel molybdate (NiMoO 4 ) directly on nickel foam (NF) through an aerosol-assisted chemical vapor deposition (AACVD) process is disclosed. The nickel molybdate nanoflowers were grown on NF by varying the deposition time for 60 and 120 min at a fixed temperature of 480° C. and their efficiency was investigated as oxygen evolution reaction (OER) catalysts in 1 M KOH electrolyte. The NiMoO 4  nanoflowers of NF obtained after 60 minutes of AACVD process showed OER performance with lowest overpotential of 320 mV to reach standard current density of 10 mA cm −2 . The catalyst continuously performed the OER for 15 h, signifying its prominent stability under electrochemical conditions.

STATEMENT OF PRIOR DISCLOSURE BY THE INVENTOR

Aspects of the present disclosure are described in M. A. Ehsan;“Aerosol-Assisted Chemical Vapor Deposition Growth of NiMoO₄ Nanoflowerson Nickel Foam as E□ective Electrocatalysts toward Water Oxidation”;Nov. 11, 2021; ACS Omega, incorporated herein by reference in itsentirety.

BACKGROUND Technical Field

This invention is related to fabrication of electrocatalysts derivedfrom transition metals and the use of the electrocatalysts in wateroxidation reactions. The fabrication of electrocatalysts includesaerosol-assisted chemical vapor deposition of certain transition metalcomplexes on a nickel substrate.

Description of the Related Art

The “background” description provided herein is for the purpose ofgenerally presenting the context of the disclosure. Work of thepresently named inventors, to the extent it is described in thisbackground section, as well as aspects of the description which may nototherwise qualify as prior art at the time of filing, are neitherexpressly nor impliedly admitted as prior art against the presentinvention.

Existing non-renewable, exhaustible energy resources are a considerablerisk to the living environment. The enormous amount of CO₂ in theatmosphere (surpassing 400 ppm) has become a major global problem thatneeds to be addressed by developing and employing sustainable andrenewable energy alternative energy sources such as hydrogen.High-energy-density, CO₂-neutral, and eco-friendly hydrogen-based fuelscan potentially serve as a versatile feedstock for the synthesis ofvaluable chemicals. In this regard, hydrogen, obtained from aphotoelectrochemical and electrochemical water splitting process, is theonly clean and economically viable energy source that is green and withzero emission. In addition, the abundant water supply ensures thesustainable production of hydrogen over long periods.

Electrochemical water splitting occurs in two reaction steps: thehydrogen evolution reaction (HER) and the oxygen evolution reaction(OER). The OER is considered more challenging than the HER, as it usesfour electrons to release O₂ and thus requires more energy to complete.Completing these reactions faster requires highly efficient andlong-lived electrocatalysts (ECs). Conventionally, noble metal catalysts(Ir/Ru oxides) were used as electrocatalysts, however, their high priceand scarcity are the major hurdles to advancing their water splittingapplications. Further, a range of electrocatalyst materials includinginexpensive and widespread transition-metal-based mono metals and binarymetal alloys/oxides, metal nitrides, transition metal chalcogenides,metal phosphides, and metal-free carbon materials, have been reportedextensively for use as electrocatalysts. However, for economicviability, despite all those advancements, some competent and modestelectrocatalytic systems, obtainable by straightforward methods andinexpensive precursors with high electroactive sites and enhancedcatalytic activity, still need to be disclosed.

SUMMARY

Aspects of this invention provide a method of making an electrocatalyst,comprising aerosol-assisted chemical vapor depositing a mixturecomprising Ni(acac)₂ and MoO₂(acac)₂ on a substrate to form NiMoO₄nanoflowers on the substrate, wherein the substrate is a nickel foam. Incertain embodiments, the NiMoO₄ nanoflowers have a crystal structure byXRD and are in a form of irregularly aggregated nanosheets.

Certain embodiments of this invention also provide a method of making anelectrocatalyst, comprising aerosol-assisted chemical vapor depositing amixture comprising Ni(acac)₂ and MoO₂(acac)₂ on a substrate to formNiMoO₄ nanoflowers on the substrate wherein the electrocatalyst has acrystalline single phase NiMoO₄ nanoflowers. In certain aspects of thisinvention, the NiMoO₄ nanoflowers have a crystalline single phase byX-ray Diffraction (XRD) and X-ray Photon Spectroscopy (XPS).

In certain other embodiments, this invention also provides a method ofmaking an electrocatalyst, comprising aerosol-assisted chemical vapordepositing a mixture comprising Ni(acac)₂ and MoO₂(acac)₂ on a substrateto form NiMoO₄ nanoflowers on the substrate, wherein the electrocatalysthas a surface of vertically aligned nanosheets assembled into the NiMoO₄nanoflowers.

Certain other embodiments of this invention provide an electrocatalystproduced by a method comprising aerosol-assisted chemical vapordepositing a mixture comprising Ni(acac)₂ and MoO₂(acac)₂ on a substrateto form NiMoO₄ nanoflowers on the substrate, wherein the substrate is anickel foam. In certain aspects of this invention, the NiMoO₄nanoflowers have a crystal structure and are in the form of nanosheets.In certain embodiments of this invention, the electrocatalyst has acrystalline single phase NiMoO₄. Further in certain other embodiments ofthis invention, the electrocatalyst has a surface that is verticallyaligned nanosheets assembled into NiMoO₄ nanoflowers. In yet otherembodiments of this invention, the NiMoO₄ nanoflowers have a crystallinesingle phase by XRD and XPS.

Aspects of this invention also provide a method of using anelectrocatalyst for water oxidation wherein the electrocatalyst is madeby a process comprising aerosol-assisted chemical vapor depositing amixture comprising Ni(acac)₂ and MoO₂(acac)₂ on a substrate to formNiMoO₄ nanoflowers on the substrate wherein the substrate is a nickelfoam and wherein, the process further comprises contacting theelectrocatalyst with an aqueous electrolyte solution having a pH of 8 to14; and applying a potential of 1.30 to 1.70 V to the electrocatalystand a counter electrode immersed in the aqueous electrolyte solution.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the invention and many of the attendantadvantages thereof will be readily obtained as the same becomes betterunderstood by reference to the following detailed description whenconsidered in connection with the accompanying drawings, wherein:

FIG. 1 shows X-Ray Diffraction (XRD) patterns of single phased NiMoO₄samples prepared for 60 and 120 min via aerosol-assisted chemical vapordeposition (AACVD).

FIG. 2 shows Field Emission Scanning Electron Microscope (FESEM) imagesof NM1(a) and NM2(b). Low resolution images ((a) and (b)); Highresolution (10 Kx) images ((a1) and (b1)) and (50 Kx) images ((a11) and(b11)).

FIG. 3A displays Energy Dispersive X-ray (EDX) spectra of NiMoO₄ filmsNM1.

FIG. 3B displays Energy Dispersive X-ray (EDX) spectra of NiMoO₄ filmsNM2.

FIG. 4 shows Energy Dispersive X-ray (EDX) elemental map analysis ofNM1.

FIG. 5A shows high resolution XPS spectrum of NM1 (Ni 2p).

FIG. 5B shows high resolution XPS spectrum of NM1 (Mo 3d).

FIG. 5C shows high resolution XPS spectrum of NM1 (O 1s).

FIG. 6A shows electrochemical characterization of NiMoO₄electrocatalysts compared to the NF substrate (polarization curves(LSVs) obtained at a scan rate of 10 mVsec⁻¹).

FIG. 6B shows electrochemical characterization of NiMoO₄electrocatalysts compared to NF substrate (comparison of overpotentialvalues of NiMoO₄ catalyst to reach benchmark current densities of 10 mAcm⁻²).

FIG. 6C shows electrochemical characterization of NiMoO₄electrocatalysts compared to NF substrate (polarization curves of bestOER activity of NiMoO₄ catalyst recorded at different scan rates of 1,5, 10, 25 and 50 mV sec⁻¹).

FIG. 6D shows electrochemical characterization of NiMoO₄electrocatalysts compared to NF substrate (tafel plot indicating the OERkinetics for NiMoO₄ catalysts).

FIG. 7A shows chronoamperometric response measured at an applied fixedpotential of 1.6 V.

FIG. 7B shows LSV polarization curves before and after stability tests.

FIG. 7C shows EIS plot for the NM1 electrode.

FIG. 8 shows FESEM images of NM1 thin film electrode after long termelectrochemical OER study. Low resolution images ((a) and (b)); Highresolution (50 Kx) image (c).

FIG. 9 shows EDX analysis of NM1 electrode after long term OER stabilitytest.

FIG. 10 shows AACVD process for synthesis of NiMoO₄ films on NFsubstrate.

DETAILED DESCRIPTION

The present disclosure will be better understood with reference to thefollowing definitions.

It will be further understood that the terms “comprises” and/or“comprising,” when used in this specification, specify the presence ofstated features, steps, operations, elements, and/or components, but donot preclude the presence or addition of one or more other features,steps, operations, elements, components, and/or groups thereof.

In the drawings, like reference numerals designate identical orcorresponding parts throughout the several views. Further, as usedherein, the words “a,” “an” and the like generally carry a meaning of“one or more,” unless stated otherwise. Also, the use of “or” means“and/or” unless stated otherwise. Similarly, “comprise,” “comprises,”“comprising” “include,” “includes,” and “including” are interchangeableand not intended to be limiting.

Furthermore, the terms “approximately,” “approximate,” “about,” andsimilar terms generally refer to ranges that include the identifiedvalue within a margin of 20%, 10%, or preferably 5%, and any valuesthere between. For example, if a stated value is about 8.0, the valuemay vary in the range of 8±1.6, ±1.0, ±0.8, ±0.5, ±0.4, ±0.3, ±0.2, or±0.1.

Disclosure of values and ranges of values for specific parameters (suchas temperatures, molecular weights, weight percentages, etc.) are notexclusive of other values and ranges of values useful herein. It isenvisioned that two or more specific exemplified values for a givenparameter may define endpoints for a range of values that may be claimedfor the parameter. For example, if Parameter X is exemplified herein tohave value A and also exemplified to have value Z, it is envisioned thatparameter X may have a range of values from about A to about Z.Similarly, it is envisioned that disclosure of two or more ranges ofvalues for a parameter (whether such ranges are nested, overlapping ordistinct) subsume all possible combination of ranges for the value thatmight be claimed using endpoints of the disclosed ranges. For example,if parameter X is exemplified herein to have values in the range of 1-10it also describes subranges for Parameter X including 1-9, 1-8, 1-7,2-9, 2-8, 2-7, 3-9, 3-8, 3-7, 2-8, 3-7, 4-6, or 7-10, 8-10 or 9-10 asmere examples. A range encompasses its endpoints as well as valuesinside of an endpoint, for example, the range 0-5 includes 0, >0, 1, 2,3, 4, <5 and 5.

As used herein, the words “preferred” and “preferably” refer toembodiments of the technology that afford certain benefits, undercertain circumstances. However, other embodiments may also be preferred,under the same or other circumstances. Furthermore, the recitation ofone or more preferred embodiments does not imply that other embodimentsare not useful, and is not intended to exclude other embodiments fromthe scope of the technology.

As referred to herein, all compositional percentages are by weight ofthe total composition, unless otherwise specified. As used herein, theword “include,” and its variants, is intended to be non-limiting, suchthat recitation of items in a list is not to the exclusion of other likeitems that may also be useful in the materials, compositions, devices,and methods of this technology.

The present disclosure further includes all isotopes of atoms occurringin the present compounds. Isotopes include those atoms having the sameatomic number but different mass numbers. By way of general example, andwithout limitation, isotopes of hydrogen include deuterium and tritium,isotopes of oxygen include ¹⁶O, ¹⁷O and ¹⁸O. Isotopically labeledcompounds of the disclosure can generally be prepared by conventionaltechniques known to those skilled in the art or by processes and methodsanalogous to those described herein, using an appropriate isotopicallylabeled reagent in place of the non-labeled reagent otherwise employed.

As defined here, an aerosol is a suspension of solid or liquid particlesin a gas. An aerosol includes both the particles and the suspending gas.Primary aerosols contain particles introduced directly into the gas,while secondary aerosols form through gas-to-particle conversion. Thereare several measures of aerosol concentration. Environmental science andhealth fields often use the mass concentration (M), defined as the massof particulate matter per unit volume with units such as μg/m³. Alsocommonly used is the number concentration (N), the number of particlesper unit volume with units such as number/m³ or number/cm³. The size ofparticles has a major influence on their properties, and the aerosolparticle radius or diameter (d_(P)) is a key property used tocharacterize aerosols. Aerosols vary in their dispersity. A monodisperseaerosol, producible in the laboratory, contains particles of uniformsize. Most aerosols, however, as polydisperse colloidal systems, exhibita range of particle sizes. Liquid droplets are almost always nearlyspherical, but scientists use an equivalent diameter to characterize theproperties of various shapes of solid particles, some very irregular.The equivalent diameter is the diameter of a spherical particle with thesame value of some physical property as the irregular particle. Theequivalent volume diameter (d_(e)) is defined as the diameter of asphere of the same volume as that of the irregular particle. Alsocommonly used is the aerodynamic diameter. The aerodynamic diameter ofan irregular particle is defined as the diameter of the sphericalparticle with a density of 1000 kg/m³ and the same settling velocity asthe irregular particle.

As used herein, nanoflowers are particles exhibiting a characteristicthree-dimensional flowerlike morphology.

The present disclosure relates to a method of producingelectrocatalysts. This method involves contacting an aerosol with asubstrate to deposit a nanostructured layer onto the substrate, therebyforming the electrocatalyst. As described here, “contacting an aerosolwith a substrate” is considered to be synonymous with “contacting asubstrate with an aerosol.” Both phrases mean that the substrate isexposed to the aerosol, so that a portion of the suspended particles ofthe aerosol directly contact the substrate. Contacting may also beconsidered equivalent to “introducing” or “depositing,” such as“depositing an aerosol onto a substrate.” In one embodiment, thecontacting may be considered aerosol-assisted chemical vapor deposition(AACVD). In one embodiment, the method of making the electrocatalyst maybe considered a one-step method, as the formation of the nanostructuredlayer takes place immediately following and/or during the contacting ofthe aerosol with the substrate.

Aspects of this invention provide a method of making an electrocatalyst,comprising aerosol-assisted chemical vapor depositing a mixturecomprising nickel and molybdenum precursors on a substrate to formnanoflowers on the substrate. The aerosol contains a carrier gas, amixture comprising nickel and molybdenum precursors, and a solvent. Inone embodiment, the aerosol consists of, or consists essentially of, acarrier gas, a mixture comprising nickel and molybdenum precursors, anda solvent before the contacting, preferably immediately before thecontacting. Preferably, the mixture comprising nickel and molybdenumprecursors is dissolved or dispersed in the solvent. In one embodiment,the mixture comprising nickel and molybdenum precursors has anacetylacetone or acetylacetonate (acac) ligand, a trifluoro-acetate(TFA) ligand, an acetate ligand (OAc), an isopropanol (^(i)PrOH) ligand,a tetrahydrofuran (THF) ligand, and/or a water (H₂O) ligand. In oneembodiment, the substrate is a metal substrate. The precursors mayinclude vanadium and cobalt in addition to the Ni and Mo. A metalsubstrate is a at least one selected from the group consisting of tin,aluminum, zinc, and nickel foam. In one embodiment, the substrate isnickel foam. In one embodiment, the nanoflowers are NiMoO₄ nanoflowers.In certain embodiments, the NiMoO₄ nanoflowers have a crystal structureby XRD and are in a form of nanosheets.

Exemplary solvents applicable to the method disclosed herein include,but are not limited to, toluene, tetra-hydrofuran, acetic acid, acetone,acetonitrile, butanol, dichloromethane, chloroform, chlorobenzene,dichloroethane, diethylene glycol, diethyl ether, dimethoxy-ethane,dimethyl-formamide, dimethyl sulfoxide, ethanol, ethyl acetate, ethyleneglycol, heptane, hexamethylphosphoramide, hexamethylphosphoroustriamide, methanol, methyl t-butyl ether, methylene chloride, pentane,cyclopentane, hexane, cyclohexane, benzene, dioxane, propanol, isopropylalcohol, pyridine, triethyl amine, propandiol-1,2-carbonate, ethylenecarbonate, propylene carbonate, nitrobenzene, formamide,γ-butyrolactone, benzyl alcohol, n-methyl-2-pyrrolidone, acetophenone,benzonitrile, valeronitrile, 3-methoxy propionitrile, dimethyl sulfate,aniline, n-methylformamide, phenol, 1,2-dichlorobenzene, tri-n-butylphosphate, ethylene sulfate, benzenethiol, dimethyl acetamide,N,N-dimethylethanamide, 3-methoxypropionitrile, diglyme, cyclohexanol,bromobenzene, cyclohexanone, anisole, diethylformamide, 1-hexanethiol,ethyl chloroacetate, 1-dodecanthiol, di-n-butylether, dibutyl ether,acetic anhydride, m-xylene, o-xylene, p-xylene, morpholine,diisopropylethanamine, diethyl carbonate, 1-pentanediol, n-butylacetate, and 1-hexadecanethiol. In one embodiment, the solvent comprisespyridine, N,N-dimethylformamide (DMF), N,N-dimethylacetamide, N-methylpyrrolidone (NMP), hexamethylphosphoramide (HMPA), dimethyl sulfoxide(DMSO), acetonitrile, tetrahydrofuran (THF), 1,4-dioxane,dichloromethane, chloroform, carbon tetrachloride, dichloroethane,acetone, ethyl acetate, pentane, hexane, decalin, dioxane, benzene,toluene, xylene, o-dichlorobenzene, diethyl ether, methyl t-butyl ether,methanol, ethanol, ethylene glycol, isopropanol, propanol, n-butanol,and mixtures thereof. In a preferred embodiment, the solvent is acetone,methanol, ethanol, and/or isopropanol. More preferably the solvent ismethanol. In one embodiment, the solvent may comprise water. The waterused as a solvent or for other purposes may be tap water, distilledwater, bidistilled water, deionized water, deionized distilled water,reverse osmosis water, and/or some other water. In one embodiment, thewater is bidistilled or treated with reverse osmosis to eliminate tracemetals. Preferably the water is bidistilled, deionized, deionizeddistilled, or reverse osmosis water, and at 25° C. has a conductivity ofless than 10 μS·cm⁻¹, preferably less than 1 μS·cm⁻¹; a resistivity ofgreater than 0.1 MΩ·cm, preferably greater than 1 MΩ·cm, more preferablygreater than 10 MΩ·cm; a total solid concentration of less than 5 mg/kg,preferably less than 1 mg/kg; and a total organic carbon concentrationof less than 1000 μg/L, preferably less than 200 μg/L, more preferablyless than 50 μg/L. In one embodiment, the carrier gas is N₂, He, Ar,and/or compressed air. Preferably the carrier gas is N₂. Preferably thesolvent and the mixture comprising nickel and molybdenum are able toform an appropriately soluble solution that can be dispersed in thecarrier gas as aerosol particles. For instance, the mixture comprisingnickel and molybdenum may first be dissolved in a volume of solvent, andthen pumped through a jet nozzle in order to create an aerosol mist. Inother embodiments, the mist may be generated by a piezoelectricultrasonic generator. Other nebulizers and nebulizer arrangements mayalso be used, such as concentric nebulizers, cross-flow nebulizers,entrained nebulizers, V-groove nebulizers, parallel path nebulizers,enhanced parallel path nebulizers, flow blurring nebulizers, andpiezoelectric vibrating mesh nebulizers.

In one embodiment, the aerosol has a mass concentration M, of 10μg/m³-1,000 mg/m³, preferably 50 μg/m³-1,000 μg/m³. In one embodiment,the aerosol has a number concentration N, in a range of 10³-10⁶,preferably 10⁴-10⁵ cm⁻³. In other embodiments, the aerosol may have anumber concentration of less than 10³ or greater than 10⁶. The aerosolparticles or droplets may have an equivalent volume diameter (d_(e)) ina range of 20 nm-100 μm, prefer-ably 0.5-70 μm, more preferably 1-50 μm,though in some embodiments, aerosol particles or droplets may have anaverage diameter of smaller than 0.2 μm or larger than 100 μm.

In one embodiment, during the contacting of the aerosol, the carrier gashas a flow rate in a range of 20-250 cm³/min, preferably 50-230 cm³/min,more preferably 75-200 cm³/min, even more preferably 100-150 cm³/min, orabout 120 cm³/min. Preferably, the aerosol has a flow rate similar tothe carrier gas, with the exception of the portion of aerosol that getsdeposited on the substrate. In one embodiment, the aerosol may enter thechamber and the flow rate may be stopped, so that a portion of aerosolmay settle on the substrate.

The contacting and/or introducing may take place within a closed chamberor reactor, and the aerosol may be generated by dispersing a solution ofthe mixture comprising nickel and molybdenum and solvent. In oneembodiment, this dispersing may be increased by fans, jets, or pumps.However, in another embodiment, an aerosol may be formed in a closedchamber with a substrate where the aerosol particles are allowed todiffuse towards or settle on the substrate. In one embodiment, theclosed chamber or reactor may have a length of 10-100 cm, preferably12-30 cm, and a diameter or width of 1-10 cm, preferably 2-5 cm. Inother embodiments, the closed chamber or reactor may have an interiorvolume of 0.2-100 L, preferably 0.3-25 L, more preferably 0.5-10 L. Inone embodiment, the closed chamber or reactor may comprise a cylindricalglass vessel, such as a glass tube.

Being in a closed chamber, the interior pressure of the chamber (andthus the pressure of the aerosol) may be controlled. The pressure may bepractically unlimited, but need not be an underpressure or anoverpressure. Preferably the chamber and substrate are heated and heldat a temperature prior to the contacting, so that the temperature maystabilize. The chamber and substrate may be heated for a time period of5 min-1 hour, preferably 10-20 min prior to the contacting.

Furthermore, the aerosol-assisted chemical vapor depositing is carriedout for from 20 to 360 min, preferably 20 to 330 min, preferably 20 to300 min, preferably 30 to 250 min, preferably 30 to 220 min, preferably30 to 200 min at a fixed temperature of 200-1000° C., preferably300-900° C., preferably 400-800° C., preferably 400-700° C. In yet otherembodiments of this invention, the aerosol-assisted chemical vapordepositing is carried out for from 30, 40, 50, 60, 70, 80, 90, 100, 110,120, 130, 140, 150, 160, 170, 180, and/or 190 to 200 min at a fixedtemperature of 400, 430, 460, 490, 510, 540, 570, 600, 630, and/or 660to 700° C.

The method of making electrocatalyst may further comprise a step ofcooling the electrocatalyst after the contacting. The electrocatalystmay be cooled to a temperature between 10 to 45° C., 20 to 40° C., or 25to 35° C. under an inert gas (such as N² or Ar) over a time period of0.5-5 h, 0.75-4 h, 1-3 h, 1.25-2.5 h, or 1.5-2 h. In one embodiment, theelectrocatalyst may be left in the chamber and allowed to cool.

In one embodiment, the precursors are Ni(acac)₂ and MoO₂(acac)₂. In oneembodiment, the electrocatalyst has a crystalline single phase NiMoO₄nanoflowers.

In one embodiment, the electrocatalyst have a surface of verticallyaligned nanosheets assembled into the NiMoO₄ nanoflowers.

In some embodiments, the nanoflowers described herein may be called“microflowers”.

In one embodiment, the nanoflowers disclosed herein is substantiallyfree of dopants, which includes being substantially free of, preferablyless than 1.0%, 0.1%, 0.01% or 0.001%, or completely free of (i.e., 0wt. %) dopants. Exemplary dopants include, but are not limited to, zinc,lithium, and vanadium.

The electrocatalyst may comprise NiMoO₄ in the form of an amorphousphase, a crystalline phase, or both. In one embodiment, theelectrocatalyst consists essentially of crystalline NiMoO₄, meaning thatthe NiMoO₄ comprises at least 99 wt %, preferably 99.9 wt %, morepreferably 99.95 wt % NiMoO₄ in a crystalline state, relative to a totalweight of the NiMoO₄.

The nanoflowers are randomly arranged on the surface of the substrate.

The substrate may be of any desirable shape, such as, a circle, atriangle, a rectangle, a pentagon, a hexagon, an irregular polygon, acircle, an oval, an ellipse, or a multilobe. Preferably, the substrateis rectangular in shape with a length and width of 0.5-5 cm, 1-4 cm, or2-3 cm, respectively. The substrate may have an area in a range of0.25-25 cm², preferably 0.5-5 cm², more preferably about 2 cm².

The nanoflowers deposited on the substrate may have an average thicknessin a range of 0.5-5 μm, preferably 0.7-4 μm, more preferably 0.8-3 μm,even more preferably 0.9-2 μm, or about 1 μm. In one embodiment, thethickness of the nanoflowers may vary from location to location on thesubstrate by 1-30%, 5-20%, or 8-10% relative to the average thickness ofthe nanoflowers deposited on the substrate. In a preferred embodiment,70-100%, more preferably 80-99%, even more preferably 85-97% of thesurface of the substrate is covered with the nanoflowers, though in someembodiments, less than 70% of the surface of the substrate is coveredwith the nanoflowers.

In one embodiment, the NiMoO₄ nanoflowers have a crystalline singlephase by XRD and XPS. In one embodiment, the nanoflowers are in a formof irregularly aggregated nanoflakes.

In one embodiment, the NiMoO₄ nanoflowers after 20 to 150 minutes,preferably 30 to 130 minutes, preferably 40 to 90 minutes of theaerosol-assisted chemical vapor depositing have a Tafel value of 20 to160 mV dec⁻¹, preferably 30 to 140 mV dec⁻¹, preferably 50 to 100 mVdec⁻¹.

In one embodiment, the NiMoO₄ nanoflowers after 40, 45, 50, 55, 60, 65,70, 75, 80 and/or 85 to 90 minutes of the aerosol-assisted chemicalvapor depositing have a Tafel value of 50, 55, 60, 65, 70, 75, 80, 85,90, and/or 50 to 160 mV dec⁻¹, preferably 60 to 150 mV dec¹, preferably70 to 130 mV dec⁻¹, preferably 95 to 100 mV dec⁻¹.

In one embodiment, the electrocatalyst has a constant current densityafter 5 to 30 hours, preferably 8 to 25 hours, preferably 10 to 20 hourswith 5-30 mA cm⁻², preferably 7-20 mA cm⁻², preferably 7-15 mA cm².

In one embodiment, the electrocatalyst has a constant current densityafter 10, 11, 12, 13, 14, 15, 16, 17, 18 and/or 19 to 20 hours with 7,8, 9, 10, 11, 12, 13 and/or 14 to 15 mA cm⁻².

In one embodiment, the process further comprising contacting theelectrocatalyst with an aqueous electrolyte solution having a pH of 8 to14; and applying a potential of 1.20 to 1.90 V, preferably 1.25 to 1.85V, preferably 1.30 to 1.80 V, preferably 1.30 to 1.70 V to theelectrocatalyst and a counter electrode immersed in the aqueouselectrolyte solution.

In one embodiment, the process further comprising contacting theelectrocatalyst with an aqueous electrolyte solution having a pH of 8,8.5, 9, 9.5, 10, 10.5, 11, 11.5, 12, 12.5, 13 and/or 13.5 to 14; andapplying a potential of 1.30, 1.35, 1.40, 1.45, 1.50, 1.55, 1.60 and/or1.65 to 1.70 V to the electrocatalyst and a counter electrode immersedin the aqueous electrolyte solution.

FIG. 1 shows the X-Ray Diffraction (XRD) patterns of single phasedNiMoO₄ samples prepared for 60 and 120 min via aerosol-assisted chemicalvapor deposition (AACVD). Both XRD patterns exhibited good crystallinityas evident from several small and long intensity peaks shown in FIG. 1 .The peaks are labelled with their corresponding reflections from whichthey are produced. The XRD fingerprints for NM1 and NM2 look similar interms of peak positions and suggest the crystallographic analogy of theproduct synthesized in the two samples. The crystalline peaks at 2θvalues of 20-25°, preferably 21-24°, preferably 22-24°, preferably23.6°, 25-26.5°, preferably 26.2°, 25-26.7°, preferably 25.5-26.7°,preferably 26.7°, 26.6-27.3°, preferably 26.6-27.1°, preferably 26.8°,27.3-28.1°, preferably 27.3-27.9°, preferably 27.3-27.6°, preferably27.4°, 27.4-31.5°, preferably 27.5-30°, preferably 27.7-29°, preferably28.8°, 29-33°, preferably 30-33°, preferably 30-32.8°, preferably31-32.6°, preferably 32.5°, 33-37.0°, preferably 33-36.0°, preferably33-35.0°, preferably 33-34°, preferably 33.9°, 34-39°, preferably35-38°, preferably 36-38°, preferably 37.2°, 35-40°, preferably 36-40°,preferably 37-40°, preferably 38-40°, preferably 39.3°, 40-41.9°,preferably 40-41.5°, preferably 40-41.0°, preferably 40-40.8°,preferably 40.6°, 40-44°, preferably 41-43°, preferably 42.0°, 42-47°,preferably 43-46°, preferably 44.2°, 44-52°, preferably 45-50°,preferably 46-49°, preferably 46-48°, preferably 47.4°, 50-54°,preferably 51-53°, preferably 52-53°, preferably 52.3°, 53-59°,preferably 53-58°, preferably 53-57°, preferably 53-56°, preferably53-55°, preferably 54.2°, 55-62°, preferably 56-61°, preferably 58-61°,preferably 60.7°, 60-64°, preferably 61-63°, preferably 62-63°,preferably 62.3°, 62-78°, preferably 63-75°, preferably 63-70°,preferably 63-65°, preferably 64.8° and 75-80°, preferably 76-89°,preferably 78.4° are indexed to the reflections (02-1), (201), (002),(220), (11-2), (31-1), (112), (31-2), (400), (040), (330), (222),(33-2), (241), (20-4), (53-1), (42-4), (44-3), (333) and (82-2)respectively, and match with the crystallographic data of single phaseNickel Molybdenum Oxide “NiMoO₄” in monoclinic crystal system (ICSD No.00-045-0142). No other crystalline nickel or molybdenum oxide phases areidentified from XRD patterns as impurity. In both XRD patterns thedominant, 100% intensity peak is situated at 2θ 23 to 30°, preferably 25to 29°, preferably 25 to 28°, preferably 26. 8°.

The surface morphology of the fabricated films developed after 30 to 200min, preferably 40 to 150 min, preferably 50-130 min, preferably 60 and120 min deposition time on nickel foam (NF), were discerned by FieldEmission Scanning Electron Microscope (FESEM) analysis and the observedmicrographs are displayed in FIG. 2 . Low resolution images, FIG. 2(a),(b) indicate that the NF strut is wreathed with a layer of crystallites.The enlarged images shown in FIG. 2 (al), (b1) display severalintertwined close packed spherical objects which flourished intoblooming flower like patterns. Further insight to these structuresreveals that the intertwined patterns are comprised of large number ofvertically arranged cross-linked nano-sheet like petals with clear grainboundaries in sample NM1 (FIG. 2 (a11)). These petal-like features werewilted when the deposition time was increased to 100 to 150 min,preferably 120 min (NM2) due to the extended sintering process (FIG. 2(b11)). These hierarchical flower-like structures comprising2D-nano-sheets play significant role in various electrochemical baseddevices such as supercapacitor, dye sensitized solar cell, hydrogenstorage and water splitting studies. The hierarchical petal networks on3D NF present high surface area and provide effective contact betweenthe catalyst material and electrolyte ions. Therefore, it is expectedthat the NiMoO₄ petals obtained after 30 to 90 min, preferably 40 to 80min, preferably 60 min of deposition time (NM1) are comprised of greaternumber of interfacial and electrocatalytic active sites that could leadto the enhanced OER performance.

The atomic composition in NiMoO₄ films was determined by energydispersive X-ray analysis (EDX). As the films were grown on NFsubstrates, it was expected to have higher Ni concentration due to thecontribution from NF substrates, therefore, the atomic concentration wasmeasured from the analogous film samples grown on plane glasssubstrates. FIG. 3 displays the EDX patterns of NiMoO₄ film samples withmeasured % atomic concentration of Ni and Mo atoms which empiricallyexist in 1 to 1 mole ratio. The elemental map analysis (FIG. 4 ) showsthe atomic dispersion of Ni and Mo depicting a uniform and evendistribution of both elements on the film surface.

The chemical behavior and oxidation state of the elements involved inNiMoO₄ film (NM1) was investigated by X-ray photoelectron spectroscopy(XPS). FIG. 5 indicates the high resolution XPS spectrum of constituentNi, Mo and O elements. The Ni 2p spectrum involves two major peaksrelated to Ni 2p_(3/2) and Ni 2p_(1/2) spin orbitals at binding energyvalue 858 and 876 eV respectively as disclosed by Hong et al. inRational Construction of Three-Dimensional Hybrid Co₃O₄@NiMoO₄Nanosheets Array for Energy Storage Application, J. Power Sources 2014,270, 516-525; the entire disclosure is incorporated herein by reference.The corresponding satellites peaks appeared at 850 to 870 eV, preferably863 and 870 to 900 eV, preferably 883 eV. This electronic structuresignifies the existence of Ni²⁺ oxidation in NiMoO₄. The Mo 5d spectrumexhibits a doublet peak of binding energies located at 220 to 235 eV,preferably 232.5 eV and 233 to 240 eV, preferably 235.6 attributed withMo3d_(5/2) and Mo 3d_(3/2), respectively as disclosed by Hussain et al.in Continuous and High Electrical Performances of Bilayer to Few LayersMoS₂ Fabricated by RF Sputtering via Post-Deposition Annealing Method.Sci. Reports 2016 61, 2016, 6 (1), 1-13; the entire disclosure isincorporated herein by reference. The difference in binding energy (ΔE)of about 3.1 eV is the typical for Mo⁶⁺ oxidation state in NiMoO₄ asdisclosed by Blomberg et al. in Bimetallic Nanoparticles as a ModelSystem for an Industrial NiMo Catalyst. Materials (Basel), 2019, 12(22); the entire disclosure is incorporated herein by reference. Thesymmetrical O1s spectrum shows two peaks at binding energy of 520 to 531eV, preferably 531 and 532 to 540 eV, preferably 531.7 eV,characteristic of metal oxygen bonds in NiMoO₄ as disclosed by Wang etal. in Hierarchical NiMoO₄ Nanowire Arrays Supported on MacroporousGraphene Foam as Binder-Free 3D Anodes for High-Performance LithiumStorage, Phys. Chem. Chem. Phys. 2015, 18 (2), 908-915; the entiredisclosure is incorporated herein by reference. The XPS observations arecomparable with the previous reports on NiMoO₄ materials. The XPSresults confirm that oxidation states of Ni, Mo and O elements is +2, +6and −2, respectively which agrees well with the chemical formula NiMoO₄identified from XRD data.

Electrochemical Water Oxidation on NiMoO₄ Nanosheets

The well-formed crystalline, hierarchical and porous nanostructures onNF can maximize the effective mass transport and boost theelectrocatalytic performance as disclosed by Aminu et al. inMultifunctional Mo-N/C@MoS₂ Electrocatalysts for HER, OER, ORR, andZn-Air Batteries, Adv. Funct. Mater. 2017, 27 (44), 1702300; the entiredisclosure is incorporated herein by reference. The NM1 and NM2containing NiMoO₄ nanosheets were investigated for theirelectrocatalytic efficiency by studying the Oxygen Evolution Reaction(OER) in alkaline media. The surface of the electrodes deposited withNiMoO₄ nanosheets were electrochemically activated employing cyclicvoltammetry (CV). The electrodes were scanned for 30 to 60 consecutivecycles, preferably 40 to 60 consecutive cycles, preferably 50consecutive cycles at a scan rate of 30 to 60 mV s⁻¹, preferably 40 to60 mV s⁻¹, preferably 50 mV s⁻¹. The peaks indicative of strongreversible redox reactions between Ni²⁺/Ni³⁺ or NiO/NiOOH were observedin both samples. The oxidative peaks for Ni oxidation (Ni⁺² to Ni⁺³)appeared between 1.3-1.5 V vs Reference Hydrogen Electrode (RHE)implying a favorable electrochemical activity for oxidation reactions asdisclosed by Jothi et al. in Enhanced Methanol Electro-Oxidation overin-Situ Carbon and Graphene Supported One Dimensional NiMoO4 Nanorods,J. Power Sources 2015, 277, 350-359; the entire disclosure isincorporated herein by reference. The voltammograms showed that theintensity of redox peaks increased with increasing the number of cyclicscans. It could be inferred that the greater number of NiOOH activesites were generated due to increased Ni⁺² oxidation with each cycle.The increase in catalytic sites resulted in improved overpotential andthe current density of the catalyst.

The linear sweep voltammetry (LSV) was performed under static conditionto measure the OER activity of NiMoO₄ nanosheets. The LSV curves aredisplayed in FIG. 6 . Both the electrodes i.e., NM1 and NM2 demonstrateda higher current density and lower overpotentials as compared to thebare NF (FIG. 6 a ). A final current density of about 170 to 185 mAcm⁻²,preferably 178 mAcm⁻² was attained at a potential 2.0 V (Vs RHE) withNM1 whereas NM2 showed a current density of about 150 to 168 mAcm⁻²,preferably 158 mAcm⁻². Moreover, electrode NM1 showed an improvedoverpotential as compared to that obtained after 80 to 200 min,preferably 100 to 150 min, preferably 120 min. To deliver a currentdensity of 5 to 20 mA cm⁻², preferably 5 to 15 mA cm⁻², preferably 10 mAcm⁻², the overpotential required was 200 to 400 mV, preferably 250 to350 mV, preferably 320 mV by NM1 whereas NM2 displayed a relatively highonset potential and required 340 to 400 mV, preferably 360 mV to achievethe current density of 5 to 15 mA cm⁻², preferably 10 mA cm⁻² (FIG. 6 b). This indicates that a better OER could be achieved by employingdeposition time of 0.5 to 3 h, preferably 1 h. The decrease inperformance of NM2 could be attributed to the crumbling of petal likemicrostructure (FIG. 2 (b11)) which might have lost the catalytic activesites to decrease its OER performance.

The effect of scan rate on the electrocatalytic efficiency of NM1 forOER was investigated by varying the scan rates (5, 10, 25, 50 and 100 mVs¹) in LSV as shown in FIG. 6 c . It is clear from the results that theoxidation peak shifted towards higher potential with an increase in thescan rate suggesting a diffusion-controlled charge-transfer mechanismwith no obvious change in final current density as disclosed by Shombeet al. in Shombe, G. B.; Khan, M. D.; Alenad, A. M.; Choi, J.; Ingsel,T.; Gupta, R. K.; Revaprasadu, N. Unusual Doping Induced PhaseTransitions in NiS via Solventless Synthesis Enabling SuperiorBifunctional Electrocatalytic Activity, Sustain. Energy Fuels 2020, 4(10), 5132-5143; the entire disclosure is incorporated herein byreference. The internal diffusion resistance within the active materialincreased leading to shift in redox peaks with increasing scan rate asdisclosed by Yin et al. in Hierarchical Nanosheet-Based CoMoO₄—NiMoO₄Nanotubes for Applications in Asymmetric Supercapacitors and the OxygenEvolution Reaction, J. Mater. Chem. A 2015, 3 (45), 22750-22758; theentire disclosure is incorporated herein by reference. Moreover, theintensities of the redox peaks changed and an increase in the currentdensity of redox peak was observed upon increasing the scan rate. Thisis indicative of surface controlled electrochemical process, i.e., thekinetics of the interface faradic redox reactions as disclosed by Yin etal. in Hierarchical Nanosheet-Based CoMoO₄—NiMoO₄ Nanotubes forApplications in Asymmetric Supercapacitors and the Oxygen EvolutionReaction, J. Mater. Chem. A 2015, 3 (45), 22750-22758; the entiredisclosure is incorporated herein by reference.

To gain further insights into the OER kinetics, Tafel slopes werederived by fitting the linear part of the polarization curves for allthe electrodes. Generally, a lower value of Tafel slope signifiessuperior catalytic activity as disclosed by Galani et al. in Developmentof RuO₂/CeO₂ Heterostructure as an Efficient OER Electrocatalyst forAlkaline Water Splitting, Int. J. Hydrogen Energy 2020, 45 (37),18635-18644; the entire disclosure is incorporated herein by reference.The Tafel slope of NM1 was estimated to be 75 mV dec⁻¹, much lower thanthat of NM2 (92 mV dec⁻¹) and the bare NF (230 mV dec⁻¹), confirming afaster OER kinetics on NM1. The improved electrocatalytic performance ofNM1 could be attributed to the high surface area and the effectiveinterfacial contact between the electrocatalyst and the electrolyte. Thehomogeneous mesoporous structure formed due to the growth of 2Dnanosheet petals on the NF provided greater number of active sitesleading to the enhanced OER performance as disclosed by Sahu et al. inMetal-Organic Framework (MOF) Derived Flower-Shaped CoSe₂ Nanoplates asa Superior Bifunctional Electrocatalyst for Both Oxygen and HydrogenEvolution Reactions, Sustain. Energy Fuels 2021; the entire disclosureincorporated herein by reference.

The electrocatalytic OER performance of the synthesized NiMoO₄nanosheets on NF by AACVD was compared with benchmark OER catalysts. TheOER performance at a current density 10 mAcm⁻², overpotential along withstability of the catalysts are presented in Table 1. The of NiMoO₄catalyst prepared by AACVD is better than benchmark OER catalystsmentioned in Table 1.

TABLE 1 Comparison of OER parameters of NiMoO₄ catalysts with benchmarkOER catalysts Over- potential Reference given (mV) @ below Electro-Synthesis 10 mA Elec- incorporated catalyst method cm⁻² trolyte hereinIrO₂ Commercial 301 1M KOH Comp. Ex. 1* RuO₂ Commercial 322 1M KOH Comp.Ex. 2 ** NiMoO₄ hydrothermal 340 1M KOH Comp. Ex. 3 *** NiMoO₄ Thermalroute 410 1M KOH Comp. Ex. 4 **** NiMoO₄ AACVD 320 1M KOH This invention* Luo, F.; Zhang, Q.; Yu, X.; Xiao, S.; Ling, Y.; Hu, H.; Guo, L.; Yang,Z.; Huang, L.; Cai, W.; Cheng, H. Palladium Phosphide as a Stable andEfficient Electrocatalyst for Overall Water Splitting, Angew. ChemieInt. Ed. 2018, 57 (45), 14862-14867.**Hyun, S.; Shanmugam, S. Hierarchical Nickel-Cobalt DichalcogenideNanostructure as an Efficient Electrocatalyst for Oxygen EvolutionReaction and a Zn-Air Battery, ACS Omega 2018, 3 (8), 8621-8630.*** Zhao, X.; Meng, J.; Yan, Z.; Cheng, F.; Chen, J. NanostructuredNiMoO4 as Active Electrocatalyst for Oxygen Evolution, Chinese Chem.Lett. 2019, 30 (2), 319-323.**** Zhang, S.; She, G.; Li, S.; Qu, F.; Mu, L.; Shi, W. Enhancing theElectrocatalytic Activity of NiMoO4 through a Post-PhosphorizationProcess for Oxygen Evolution Reaction, Catal. Commun. 2019, 129, 105725.

To determine feasibility of their practical application, and to testelectrochemical stability of the developed electrodes, the long-termelectrochemical stability of the developed electrode NM1 wasinvestigated by employing chronoamperometry in 0.5 to 2 M, preferably 1M KOH solution. The chronoamperometric test was performed by applyingpotential of 1.57 V (Vs RHE) and the current density was monitored for aperiod of 10-20 h, preferably 15 h as shown in FIG. 7 a . The catalystdisplayed a constant and steady current for the investigated period andthe measured current density after 10-20 h, preferably 15 h was 8 to 11mA cm⁻², preferably 9.7 mA cm⁻² i.e., a drop of only 1-10%, preferably1-5%, preferably 3% was observed. After the long-term stability test,polarization curve for used NM1 was recorded and compared with its freshform as indicated in FIG. 7 b . Both polarization curves are almostidentical suggesting the stable OER performance of the NM1 electrodeeven after long term electrochemical studies.

Furthermore, electrochemical impedance spectroscopy (EIS) was performedto measure the internal resistance and charge transfer resistance at theelectrode/electrolyte interface. The Nyquist plots of the synthesizedmaterials are shown in FIG. 7 c . The charge transfer resistance (Rct)is closely related to the electrocatalytic kinetics and can bedetermined by measuring the diameter of the semicircle. From the Nyquistplots, it is clear that the diameter of the semicircle for NM1 issmaller as compared to NM2 and bare NF. The smaller diameter indicatesthat NM1 has the minimum charge transfer resistance and faster electrodedynamics as compared to other NM2 sample. The Rct value of NM1 was foundto be lower than that of NM2 and bare NF.

The surface of the NM1 was analyzed after long term chronopotentiometrystability test and images are shown in FIG. 8 . The FESEM images at lowand high magnification reveled flake like structure and the footprintsof these flakes show some resemblance with the initial flower likemorphology of unused NM1 electrode (FIG. 2 c ). This confirms that thethin film electrode after prolong stability has retained its structurewith minimal deterioration.

FIG. 9 . shows the corresponding EDX spectrum of NM1. Both key elementsNi, and Mo are present on the surface of used NM1; K atoms are includedfrom KOH electrode. It is difficult to predict the exact concentrationof Ni atoms due to the contributions from nickel foam substrate.However, the % atomicity of Mo atoms is found to be 4 to 10%, preferably6.65% which is almost half of the initial % atomic concentration of Mo(13.49%) found in the NM1 electrode before electrochemicalinvestigation. The decrease in Mo atomic concentration suggests that Moatoms are leached out during the OER process, which is a common reportedphenomenon (Dürr, R. N.; Maltoni, P.; Tian, H.; Jousselme, B.;Hammarström, L.; Edvinsson, T. From NiMoO4 to γ-NiOOH: Detecting theActive Catalyst Phase by Time Resolved in Situ and Operando RamanSpectroscopy, ACS Nano 2021, 15 (8), 13504-13515; and Sagar Ganguli;Sourav Ghosh; Soumik Das; Venkataramanan Mahalingam. Inception ofMolybdate as a “Pore Forming Additive” to Enhance the BifunctionalElectrocatalytic Activity of Nickel and Cobalt Based Mixed Hydroxidesfor Overall Water Splitting, Nanoscale 2019, 11 (36), 16896-16906; bothdisclosures incorporated herein by reference).

In this study the fabrication of 2D nanoflowers of NiMoO₄ on nickel foamby AACVD approach has been demonstrated. The NiMoO₄ nanosheets growninto flower like structures with each sheets representing petals ofindividual flowers. The AACVD process duration has profound effect onparticle morphology as well as electrocatalytic activity for oxygenevolution reaction. The film grown for 60 min showed hierarchicalcrystalline ultrafine and highly porous, thin films with well-definedgrain boundaries. However, an increased AACVD duration, i.e., 120 minhad a detrimental effect on morphology of the NiMoO₄ structures andhence the electrocatalytic performance. The NiMoO₄ catalyst obtainedafter 60 min of AACVD deposition exhibited relatively smalloverpotential value to attain current density of 10 mAcm⁻². Theoverpotential required to this current density was 320 mV and 360 mV forthe films obtained after 60 and 120 min, respectively. The Tafel plotvalue also supported a higher OER kinetics which was corroborated by thelow charge resistance shown by EIS measurement. In addition, thecatalyst was durable and sustainable enough to continuously catalyze theOER for 15 h. This suggests that transition metal based electrocatalystscould be fabricated by simple AACVD approach that could be a potentialreplacement for the noble-metal based electrocatalysts for oxygenevolution reaction. As used herein, the term “noble metal” is a metalselected from the group consisting of Ru, Rh, Pd, Ag, Os, Ir, Pt, andAu.

Examples

Materials and Methods

Nickel(II) acetylacetonate ((Ni(acac)₂), molybdenum diacetylacetonatedioxide MoO₂(acac)₂ and methanol were obtained from Sigma Aldrich. TheNF substrate of thickness 0.9 mm and porosity 93% was obtained from Goodfellow global supplier for materials.

Nickel Molybdenum Oxide Thin Film Fabrication

Solid solution NiMoO₄ thin films were fabricated on NF substrates usingaerosol assisted chemical vapor deposition (AACVD) as shown in FIG. 10 .The feedstock solution was prepared prior to AACVD process. The detailsof the setup and working principle of AACVD is also elaborated in Joya,K. S.; Ehsan, M. A.; Babar, N. U. A.; Sohail, M.; Yamani, Z. H.Nanoscale Palladium as a New Benchmark Electrocatalyst for WaterOxidation at Low Overpotential, J. Mater. Chem. A 2019, 7 (15),9137-9144; the entire disclosure is incorporated by reference. Briefly,the precursors, 100 mg (0.4 mmol) of Ni(acac)₂ and 126 mg (0.4 mmol),MoO₂(acac)₂ were dissolved in 20 mL of methanol at room temperature andthe solution was stirred continuously. The yellowish green solutionobtained after 30-minute stirring acted as feedstock for thin filmsdeposition. Films were prepared by altering the deposition time for 60and 120 min at a fixed temperature of 480° C. The liquefied precursorwas then converted into gaseous stream with the help of ultrasonicgenerator and delivered to the reactor tube, fitted in horizontal tubefurnace, with aid of carrier gas (N₂, 99.99% purity). The temperature oftube furnace was set at 480° C. and NF substrate was positioned in sucha way that precursor mist directly landed on its surface. The precursorcloud decomposed on the heated NF surface to form thin films. The AACVDprocess for different time periods was carried out to obtain sampleswith different microstructure. After completing the deposition process,samples were cooled under the flow of N₂ gas until furnace temperaturereached to 50° C. The grey films obtained after 60 and 120 min ofdeposition were designated as NM1 and NM2, respectively.

Thin Film Characterization

The crystallinity and phase structure of the NiMoO₄ films was revealedby powder XRD analysis recoded on a benchtop X-ray diffractometer(Rigaku MiniFlex X-ray diffractometer, Japan) using Cu Kα1 radiation(α=0.15416 nm). The surface morphology of the films were analyzed on afield emission scanning electron microscope (FESEM, TESCAN Lyra). Theelemental compositions were determined with energy dispersive X-rayanalysis (EDX, Oxford Instruments). The chemical behavior and oxidationstate of the elements involved in NiMoO₄ film was investigated by X-rayphotoelectron spectroscopy (XPS).

Electrochemical OER Studies

The prepared electrodes were evaluated on an Autolab Potentiostatsupported by NOVA 2.0 software. The electrochemical measurements wereconducted in a three-electrode cell using 1 M KOH electrolyte. Platinum(Pt) and saturated calomel electrodes (SCE) served as the counter andreference electrodes, respectively. The measured potential against thereference electrode was converted to RHE scale using the equation (1).E_(RHE)=E_((Hg/HgCl))+0.0591pH+E⁰ _((Hg/HgCl))  (1)

The Cyclic Voltammetry was performed at a scan rate of 50 mV s⁻¹. TheOER performance was measured by linear sweep voltammetry (LSV) at 10mVs⁻¹ scan rate. Chronoamperometric data was recorded at a constantapplied potential of 1.57 V vs RHE for 15 h for the durability test.

Numerous modifications and variations of the present invention arepossible in light of the above teachings. It is therefore to beunderstood that within the scope of the appended claims, the inventionmay be practiced otherwise than as specifically described herein.

The invention claimed is:
 1. A method of making an electrocatalyst,comprising: aerosol-assisted chemical vapor depositing a mixturecomprising Ni(acac)₂ and MoO₂(acac)₂ on a substrate to form NiMoO₄nanoflowers on the substrate, wherein the substrate is a nickel foam. 2.The method of making the electrocatalyst of claim 1, wherein the NiMoO₄nanoflowers are crystalline by XRD, wherein the nanoflowers are in aform of irregularly aggregated nanoflakes.
 3. The method of making theelectrocatalyst of claim 1, wherein the aerosol-assisted chemical vapordepositing is carried out for from 30 to 200 min at a temperature of 400to 700° C.
 4. The method of making the electrocatalyst of claim 1,wherein the aerosol-assisted chemical vapor depositing is carried outfor from 60 to 120 min at a temperature from 430 to 540° C.
 5. Themethod of making the electrocatalyst of claim 1, wherein theelectrocatalyst has crystalline single phase NiMoO₄ nanoflowers.
 6. Themethod of making the electrocatalyst of claim 1, wherein theelectrocatalyst has a surface of vertically aligned nanosheets assembledinto the NiMoO₄ nanoflowers.
 7. The method of making the electrocatalystof claim 1, wherein the NiMoO₄ nanoflowers have a crystalline singlephase by XRD and XPS.
 8. The method of making the electrocatalyst ofclaim 1, wherein the NiMoO₄ nanoflowers have a surface that isvertically aligned nanosheets.
 9. The method of making theelectrocatalyst of claim 1, wherein the NiMoO₄ nanoflowers after 40 to90 minutes of the aerosol-assisted chemical vapor depositing have aTafel value of 50 to 100 mV dec⁻¹.
 10. The method of making theelectrocatalyst of claim 1, wherein the electrocatalyst has a constantcurrent density after 10 to 20 hours with 7-15 mA cm⁻².